1. Field of the Invention
This invention relates to an apparatus for driving a cathode-ray tube (which will be abbreviated hereinafter as a CRT).
2. Description of the Prior Art
A CRT is widely used in apparatus such as television receivers and display units displaying process states, patterns, characters, etc. It is well known that, in order to display an image on a CRT, a driving apparatus is required for amplifying an image input signal and generating electron beams from the cathodes (the electron guns) of the CRT. Heretofore, efforts have been made for the purpose of removing noises from the input image signal applied to the CRT driving apparatus and also for the purpose of controlling the intensity of the electron beams with high fidelity in response to the level of the input image signal applied to the CRT driving apparatus. However, no one has taken notice of the difference in the power consumption of the CRT driving apparatus due to the difference in the cathode driving characteristics in individual CRT's, and none of the prior art efforts have been directed to minimization of the total power consumed by the CRT driving apparatus.
For a better understanding of the present invention, the structure of a prior art CRT driving apparatus will be briefly described before describing the present invention in detail.
FIG. 1 shows the structure of one form of prior art CRT driving apparatus. Referring to FIG. 1, a color CRT 1 includes three cathodes K, a first grid G.sub.1, a second grid G.sub.2, a third grid G.sub.3 and an anode A. Three driving transistors 11, each of which is connected at its collector to one end of a resistor 21, are connected at their collectors to the cathodes K respectively. The resistors 21 are connected at the other end thereof to a cathode drive power source E.sub.V. The first grid G.sub.1 is grounded, while the other grids G.sub.2, G.sub.3 and the anode A are connected to biasing power sources E.sub.G.sbsb.2, E.sub.G.sbsb.3 and E.sub.A respectively. There is a stray capacitor C.sub.s between the cathodes K and ground, that is, between each cathode and each of the first grid G.sub.1 and the heaters (not shown).
FIG. 2 shows cathode driving characteristics in the CRT 1 of such a structure as mentioned above, by way of example. It will be seen that the characteristics A and B are different from each other, and such a difference is attributable to the manufacturing tolerance of the cathodes K of the CRT 1. When the cathode driving characteristic in the CRT 1 is represented by the curve A, it will be seen that the cathode drive voltage is 32 volts for displaying a bright image with luminance of 225 .gamma.1x, and the voltage or the so-called cathode cutoff voltage is 73 volts for displaying a dark image with luminance of the order which can be barely sensed in a dark room or with the luminance corresponding to the so-called cutoff point. When, on the other hand, the cathode driving characteristic in the CRT 1 is represented by the curve B, it will be seen that the cathode drive voltage is 109 volts for displaying a bright image, and the cathode cutoff voltage is 159 volts.
It will be apparent from the above description that the driving circuit is required to provide various voltages from 32 volts to 159 volts since the CRT 1 includes cathodes K which may have the driving characteristic either A or B of FIG. 2, or a characteristic intermediate therebetween, when such cathodes K are to be driven from the single driving circuit. Actually, such characteristics variations are inevitable due to the manufacturing tolerance of the cathodes of CRT's. Thus, in the case of the prior art CRT driving apparatus, the cathode drive power source E.sub.V is required to provide a voltage higher than 159 volts.
On the other hand, the resistance value of each of the resistors 21 for applying the drive voltage to the respective cathodes K is limited to about 1 k.OMEGA. in order to minimize level variation of the image signal (whose frequency components range from the DC level to about 30 MHz) due to the presence of the stray capacitor C.sub.s (whose capacitance is about 10 pF).
Therefore, the prior art CRT driving apparatus has been defective in that an attempt to display an image of good quality by minimization of level variation of the image signal results in a great increase in the amount of power consumed by the resistors 21. Suppose that the voltage of the drive power source E.sub.V is 159 volts, and the resistance value R.sub.21 of each of the resistors 21 is 1 k.OMEGA.. Then, the power P.sub.R consumed by each of the resistors 21, when the cathode driving characteristic is represented by the curve A and when a bright image is displayed with V.sub.K =V.sub.D =32 volts, is given by the following equation (1): ##EQU1##
The power P.sub.T consumed by the associated transistor 11 is given by ##EQU2## Therefore, the power P consumed by one of the resistors 21 and associated transistor 11 in the driving circuit connected to the cathodes is the sum of the values provided by the equations (1) and (2) and is therefore given by ##EQU3## It can thus be seen that a considerable amount of power is consumed in the prior art CRT driving circuit.
It will be added herein for reference that the CRT driving apparatus shown in FIG. 1 is illustrated in a book entitled "Color Television Textbook (Volume 1)", particularly pp. 140-142, and published by Japan Broadcasting Corporation (NHK).